Tubing for heat exchangers operated at high temperatures and pressures



July 19, 1949.

Fis. 1

. y Summer y Cur: Keller Patented July 19, 1949 UNITED. STATES Pri'rlslv'r OFFICE 2,476,666 TUBING FOR HEAT EXCHANGERS OPER- ATED AT PRESSURES HIGH TEMPERATURES AND Curt Keller, Kusnacht, switzerland. assigner to Aktiengesellschaft Fuer Technische Studien, Zurich, Switzerland, a corporation of Switzerland Application January 14, 1946, Serial No. 641,108

(Cl. 13B-38) 3 Claims.

- This inventionl relates to heat transfer tubes for use under conditions in which the exterior of the tube must have good resistance to corrosion by hot iiue gases and slag and the tube must have good creep resistance at high temperatures.

The tube is intended for use in heaters forming a part of closed circuit power plants of the type described in appiicants Patent No. 2,172,910,

issued on September 12, 1939.

The working medium is usually air which passes through the heater under pressures ranging from 600 to 1000 p. s. i. and leaves the heater at temperatures as high -as 1300 F. Thus the pounds in the products of combustion of coals and many oils and also are attackedactively by slags and ash particles.

There are, however. plain chrome alloy steels -which resist such corrosion.' A satisfactory -chrome content is 2030%. Thev expense of welding a surface layer of chrome alloy steel to a chrome-nickel alloy steel core is heavy and indeed almost prohibitive. The present invention is based on the discovery that this not necessary. A separate envelope can be made to give excellent heat transfer and has an added advantage in that it can be replaced. The concept stems from observation of the fact that suitable chrome alloy steels4 have coemcients of thermal expansion only about half those of suitable chrome-nickel steels.

The basic concept is a tubular core of creepresisting steel with a thin envelope of corrosionresisting steel" closely encircling the core at ordinary temperatures and having a lower thermal coeiicient of expansion, so that at the high Working temperature the envelope is stressed into close contact with the core, assuring good heat transfer.

As a further detail the envelope is Wrapped on the core, either inthe form of a longitudinal strip whose edges are seamed or locked together, or in the form of a strip helically wound with a slight lap at the edges. 'I'he described procedures are preferred because it is difcult to telescope together a preiormed'tubular core and preformed tubular envelope and secure the desired close fit. Even i! Athis can be done ori a commercial basis, the life of the envelope is the life of the tube.

To afford a better understanding of the invention, typical constructions will be described by l reference to the accompanying drawing in which:

Fig. 1 isan elevation'of a tube having a longitudinally seamed envelope.

Fig. 2 is a transverse section thereof.

Fig. 3 is an elevation of a tube having' a'helically wound envelope.

Fig. 4 is a transverse section of the tube shown in Fig. 3.

In all the figures the part 6 is a .tubular core of creep resisting alloy steel, for example chromenickel alloy steel of the composition suggested above and of a thickness appropriate to the internal pressure to be encountered and to the tube l diameter.

In Figs. V1 and 2 the envelope l is of corrosion resisting chrome alloy steel', stretched in tension around the core and longitudinally seamed at 8. 'I'he strip is approximately 0.01 inch 'in thickness. An ordinary lock seam is indicated, but details of the seam may be varied.

In Figs. 3 and 4 the envelope 9 is a strip of chrome alloy steel of similar thickness wound helically under tension, successive convolutions overlapping slightly as shown at li. To prevent unwinding it is desirable to tack weld the helically wound strip at intervals as indicated at l2.

The intervals are such as to allow the strip to accommodate itself to minor exures of the core.

If either type of envelope burns out, it can be stripped from the core and a new envelope can be applied without dismantling the heater.

In any heater in which tubes are expanded in headers or welded therein. the possibility of applying the envelope to cores after the cores are xed in the headers is a very useful feature of the invention..

What is claimed is: 1. A lheat transfer tube for use in heaters for :gases under pressure in which the wall tempera- 2,476,660 3 4 thermal expnsln materially lower than that 0f REFERENCES CITED the core, whereby the envelope ls stressed at the operating temperatures and good hear, transfer The following references are of record ln the 1s assured, flle ot this patent:

2. A tube as dened in claim' 1 in which the 5 envelope is a strip tightly wrapped upon the core UNITED STATES PATENTS when cold and having a longitudinal locked seam. Number Name Date 3. A tube as defined 1n claim 1 m which the 432-059 Maschmeyer July 15,1890 envelope ls a strip wound hellcally on the core, 1,339,964 Harvey Jan. 5, 1932 and under tension when cold, the edges of said 10 139,533 Bket Dec. 6, 1938 2,258,564 Armstrong Oct. "I, 1941 strip having an overlap.

CURT KELLER. 

